Effects of Calcination Temperature and Calcination Atmosphere on the Performance of Co₃O₄ Catalysts for the Catalytic Oxidation of Toluene

Round 1

Reviewer 1 Report

The manuscript entitled “Catalytic oxidation of Co₃O₄ nanocatalysts derived from Co-MOF for Toluene: effect of calcination temperature and calcination methods”. The authors use a wide range of characterization techniques to describe the structure of the synthesized materials and to establish links between this structure and the results of the catalytic activity. However, the description of the methods and especially the discussion of the results leaves many gaps that require further discussion and the extraction of more details from the data obtained. I set out below my considerations on how to achieve this task.

First, the title. The manuscript is not about the catalytic oxidation of Co₃O₄, but about the catalytic oxidation of toluene. The title is completely badly written and that leaves a very bad first impression.

Introduction

The sentence between lines 37-41 is incorrectly worded. It has an implicit contradiction.

The affirmations made in sentences between lines 46-50 should be supported by literature references.

Please, in line 51 correct the phrase “Co3O4 catalysts catalyst”.

Line 62 says: “Co-MOF have good catalytic effect on toluene.”. On toluene what, oxidation, synthesis, cracking, decomposition? Please, correct this.

Please, use capital letter in the last name “Lei” mentioned in lines 62 and 69.

Line 68, there seems to be a misconception where it says "catalyst size". Do the authors mean the size of the metallic particle?  

Line 69 says: “resulting in the difference of catalytic activity”, is difference this positive, i.e. means an increase in the catalytic activity? Or should say “loss of catalytic activity”?

Line 76. What is “Mn-100-Ar-O”? Is a nomenclature used by the authors of reference 22 in their article? In this case should not be used in this manuscript, could cause confusion.

What do the authors mean when they mention "calcination method"? The only mention of a parameter other than temperature is the atmosphere or gas to be used in the process. Does the calcination method include other parameters such as heating ramp or dwell times at a certain temperature? If so, these should be included and discussed in the introduction.

The authors should unify the term used to refer to what they want to achieve with the toluene molecule, an oxidation, a degeneration? They do not mean the same thing.

 

Experimental section.

Line 93. The acronym of benzenetricarboxylic acid should be stated when first mentioned.

Line 104. Please, replace de word “ramping” by “heating”.

Please, declare the final particle size distribution of the catalysts after calcination.

The sentence between lines 105 and 108 should be completely rewritten, it is extremely difficult to understand.

Line 111. Please, declare the brand of the TGA device, “German” seems to be related with the origin of the company, not the brand.

Please, declare the N₂:O₂ ratio during the TGA experiment and the maximum temperature reached.

Line 112. Please replace “temperature rate” by “heating rate”.

Please, declare the conditions of sample preparation for SEM analysis and XPS analysis.

Please, declare the conditions used to perform the TPR experiment.

Why was Co₃O₄-N₂-O₂ choose to be used in the FTIR analysis among the other samples?

Please, declare the quantity of toluene introduced to saturate the sample.

Please, improve the description of the FTIR experiment. Was the sample placed inside a cell? Which kind of cell? Which resolution was used? Absorbance, transmittance?  

Line 124. When the authors say “saturate the adsorption” mean “saturate the sample”. Seems to be a misconception.

Please, describe the gasification chamber used to feed the toluene.

How did the authors ensure that there was no channeling in the catalytic bed? What is the height of the 100 mg catalytic bed in a 2 cm diameter reactor?

Please, declare the details of the GC detectors and columns.

Please, declare the units of CO₂ concentration.

 

Results

The comparison of peak intensity should be done using normalized intensities. Is not really obvious that the peak intensity of Co₃O₄ is larger in samples prepared at higher temperature. The particle size of metal oxide cluster could be estimated from the XRD patterns as well.

The affirmation made in lines 168-169 about the influence of calcination conditions on pore structure should be discussed and supported by the results.

The plots of pore size distribution should be included in the Supplementary Information together with the isotherms.

Line 179-180. Which result support the affirmation “the pore surface was found to gradually became smooth and also showed signs of expansion”? What is the “pore surface”?

Table 2 and 3. The Co³⁺/Co²⁺ and O_ads/O_latt ratios should not have units.

Why the surface composition of the samples Co₃O₄-550 and Co₃O₄-450 is not in the results?

Line 241. Which XRD results are consistent with the particle size increasing?

The comparison made in Figure 5c and 5d must be preform using reaction rate as the parameter to stablish the efficiency of each catalyst. Is a mistake to use the conversion to compare the materials.

Line 261-262. To support the affirmation “the internal grains of the catalyst tended to agglomerated” the authors should include the particle size of clusters in the catalysts.

Line 270. The comparison among the catalysts could be done in a plot of the T_50% and/or T_90% to make it easy visually.

Line 310. Please, write “stream” instead “steam”.

The apparent activation energies could be easily calculated from the data. Its values should be included in the discussion.

Why Co₃O₄-P was not included in any previous characterization? At least in XRD to confirm the Co phases.

There are no results to confirm that the mechanism fit to a L-H model. To do that, the authors must fit the experimental data to all the three models mentioned and show, mathematically, the better fit to L-H model.

How the authors are sure that “benzaldehyde, benzoic acid, bicarbonate and carbonate species were intermediate products in the degradation process of toluene” and not just spectators in the catalyst surface?

Figure 7. The steps shown lack experimental support and the methodology used to show them is not adequate. As a suggestion to the authors I leave this reference:

Vannice, M. A., & Joyce, W. H. (2005). Kinetics of catalytic reactions (Vol. 134). New York: Springer.

 

As conclusion, the authors must compare their results with literature. This is missing in the manuscript.

Author Response

Response to Reviewers

Dear editor and revewers:

Thank you very much for giving us the opportunity to revise our manuscript. We appreciate the reviewers for your constructive comments and suggestions on our article entitled " Catalytic oxidation of Co₃O₄ nanocatalysts derived from Co-MOF for Toluene：effect of calcination temperature and calcination methods" (processes-2274190).

We have studied the comments carefully and made corrections that we hope meet with approval. The main corrections have been marked in red in the revised manuscript. Now, we will reply the questions point by point.

 

Reviewers' comments:

 

Reviewer #1: The manuscript entitled “Catalytic oxidation of Co₃O₄ nanocatalysts derived from Co-MOF for Toluene: effect of calcination temperature and calcination methods”. The authors use a wide range of characterization techniques to describe the structure of the synthesized materials and to establish links between this structure and the results of the catalytic activity. However, the description of the methods and especially the discussion of the results leaves many gaps that require further discussion and the extraction of more details from the data obtained. I set out below my considerations on how to achieve this task

1. First, the title. The manuscript is not about the catalytic oxidation of Co₃O₄, but about the catalytic oxidation of toluene. The title is completely badly written and that leaves a very bad first impression.

Reply: Thanks for your advice. We have revised the title of the paper, the revised title in Lines 1-2, Page 1, “Effect of calcination temperature and calcination atmosphere on the performance of Co₃O₄ catalysts for the catalytic oxidation of toluene”.

2. The sentence between lines 37-41 is incorrectly worded. It has an implicit contradiction.

Reply: Thanks for your advice. We’ve changed lines 37-41 to lines 19-22, Page 2, “Although noble metals catalysts exhibit outstanding performance for catalytic removal of VOCs at low temperature, their susceptibly poisoning and high cost have limited their widely applications. Therefore, transition metal oxides have been considered as the alternative for VOCs oxidation due to their relatively low cost, superior reducibility and anti-poisoning ability.”.

3. The affirmations made in sentences between lines 46-50 should be supported by literature references.

Reply: Thanks for your advice. We’ve changed lines 46-50 to lines 1-10, Page 3, “Co₃O₄ has the advantages of low cost and environmental friendliness among non-noble metal oxide catalysts. In related studies, Co₃O₄ catalysts showed excellent catalytic activity in the degradation process of toluene, and the high activity of Co₃O₄ catalysts is related to the presence of mobile oxygen in their spinel-type structure, which made it have good low-temperature reduction ability and a large number of oxygen vacancies and adsorbed oxygen content on the surface. However, the specific surface area size, pore structure and physical and chemical properties of Co₃O₄ catalysts also play an important role in the degradation of toluene. Therefore, most research are devote to the synthesis of Co₃O₄ catalysts with different structures by different preparation methods.”

4. Please, in line 51 correct the phrase “Co₃O₄ catalysts catalyst”

Reply: Thanks for your advice. We’ve changed lines 51 to lines 10, Page 3. “Co₃O₄ catalysts”.

5. Line 62 says: “Co-MOF have good catalytic effect on toluene.”. On toluene what, oxidation, synthesis, cracking, decomposition? Please, correct this.

Reply: Thanks for your advice. We’ve changed lines 62 to lines 20-21, Page 3, “It has been demonstrated that Co₃O₄ catalysts derived from Co-MOF showed good degradation of toluene”.

6. Please, use capital letter in the last name “Lei” mentioned in lines 62 and 69.

Reply: Thanks for your advice. We’ve changed lines 62 and 69 to lines 22, Page 3 and lines 8, Page 4, “Juan Lei”.

7. Line 68, there seems to be a misconception where it says "catalyst size". Do the authors mean the size of the metallic particle?  

Reply: Thanks for your advice. We’ve changed lines 68 to lines 6, Page 4, “which leads to larger size of the metallic particle”.

8. Line 69 says: “resulting in the difference of catalytic activity”, is difference this positive, i.e. means an increase in the catalytic activity? Or should say “loss of catalytic activity”?

Reply: Thanks for your advice. We’ve changed lines 69 to lines 7, Page 4. “resulting in reduced catalytic activity of the catalyst”.

9. Line 76. What is “Mn-100-Ar-O”? Is a nomenclature used by the authors of reference 22 in their article? In this case should not be used in this manuscript, could cause confusion.

Reply: Thanks for your advice. We’ve changed lines 76 to lines 15, Page 4, “and found the catalytic activity of Mn₂O₃ catalyst pretreated with Ar and then with O₂”.

10. What do the authors mean when they mention "calcination method"? The only mention of a parameter other than temperature is the atmosphere or gas to be used in the process. Does the calcination method include other parameters such as heating ramp or dwell times at a certain temperature? If so, these should be included and discussed in the introduction.

Reply: Thanks for your advice. We have amended the entire text by replaced “calcination method” with “calcination atmosphere”

11. The authors should unify the term used to refer to what they want to achieve with the toluene molecule, an oxidation, a degeneration? They do not mean the same thing.

Reply: Thanks for your advice. We’ve changed “for toluene oxidation” to “for the degradation of toluene” in lines 1, Page 5.

12. Line 93. The acronym of benzenetricarboxylic acid should be stated when first mentioned.

Reply: Thanks for your advice. The full names of BTC have been added in their first appearance in lines 8, Page 1. “(BTC = 1,3,5-benzenetricarboxylic acid)”

13. Line 104. Please, replace de word “ramping” by “heating”

Reply: Thanks for your advice. We’ve changed “ramping” to “heating” in lines 2, Page 6.

14. Please, declare the final particle size distribution of the catalysts after calcination

Reply: Thanks for your advice. We have added a plot of the particle size distribution of the catalysts to the Supplementary data, which can be viewed in Fig. S2 (c).

15. The sentence between lines 105 and 108 should be completely rewritten, it is extremely difficult to understand.

Reply: Thanks for your advice. We’ve changed lines 105 and 108 to Lines 3-7, Page 6, “Then, the Co-BTC was calcined at 450 °C N₂ for 2 h with the heating rate of 2 °C·min⁻¹ to obtained Co₃O₄-N₂. And the sample of Co-BTC was first treated at 450 °C N₂ for 1 h and then treated at 350 °C O₂ for 1 h, which was named as Co₃O₄-N₂-O₂. As a comparison 0.012 mol Co(NO₃)₂·6H₂O was treated at 450 °C N₂ for 1 h and then treated at 350 °C air for 1 h to obtained Co₃O₄-P.”.

16. Line 111. Please, declare the brand of the TGA device, “German” seems to be related with the origin of the company, not the brand.

Reply: Thanks for your advice. We’ve changed lines 111 to lines 9-12, Page 6, “The thermal stability of Co-BTC was tested by a German STA-449 F4 Jupiter thermogravimetric analysis (TGA), the weight change of the catalyst was recorded from room temperature to 750 °C under air and N₂ condition, respectively, with a heating rate of 2 °C·min⁻¹.”.

17. Please, declare the N₂:O₂ratio during the TGA experiment and the maximum temperature reached.

Reply: Thanks for your advice. We have added the corresponding description in line 10-12 of page 6. “the weight change of the catalyst was recorded from room temperature to 750 °C under air and N₂ condition, respectively, with a heating rate of 2 °C·min⁻¹”.

18. Line 112. Please replace “temperature rate” by “heating rate”.

Reply: Thanks for your advice. We’ve changed “temperature rate” to “heating rate” in Lines 11-12, Page 6.

19. Please, declare the conditions of sample preparation for SEM analysis and XPS analysis.

Reply: Thanks for your advice. We have added the corresponding description in line 19-22 of page 6. “ZEISS Sigma 300 scanning electron microscope (SEM) was operated at an accelerating voltage of 15 kV and the catalyst was deposited on a clean silicon wafer and mounted on the aluminum holder by carbon tape and sputter coated with gold to minimize sample charging” and in line 1-4 of page 7. “X-ray photoelectron spectroscopy (XPS) analysis was performed on an Thermo Fisher Scientific spectrophotometer with Al Kα(1486.6 eV) as an X-ray excitation source. The survey and narrow scans were done at pass energies of 100 eV and 30 eV, respectively. The C1 s was taken as reference and assigned binding energy of 284.8 eV.”

20. Please, declare the conditions used to perform the TPR experiment.

Reply: Thanks for your advice. We have added the corresponding description in line 6-9 of page 7. “Before the H₂-TPR experiment, 0.1 g of catalyst was pretreated under Ar condition at 150 °C for 1 h. Then, the temperature was cooled down to 50 °C, the reduction process began and proceeded in the temperature range of 50−800 °C in 10 vol % H₂/Ar (30 mL·min⁻¹).”

21. Why was Co₃O₄-N₂-O₂choose to be used in the FTIR analysis among the other samples?

Reply: Thanks for your advice. Co₃O₄-N₂-O₂ has the most excellent toluene degradation activity, so we chose Co₃O₄-N₂-O₂ for FTIR analysis. We have added the corresponding description in line 21-22 of page 17. “Co₃O₄-N₂-O₂ with the best catalytic activity was used as the catalyst.”

22. Please, declare the quantity of toluene introduced to saturate the sample.

Reply: Thanks for your advice. We have added the corresponding description in line 14 of page 7. “(200 ppm)”

23. Please, improve the description of the FTIR experiment. Was the sample placed inside a cell? Which kind of cell? Which resolution was used? Absorbance, transmittance?

Reply: Thanks for your advice. We have added the corresponding description in line 11-16 of page 7. “the catalyst was squeezed into a thin slice in the DRIFT cell and tested in situ using absorbance, activated to remove surface impurities under Ar for 1 h at 200 °C, and then cooled down to room temperature. Toluene (200 ppm) was introduced until the peaks remained unchanged. After that, the Ar or air to record the changes in the IR peaks at different temperatures under different gas atmospheres.”

24. Line 124. When the authors say “saturate the adsorption” mean “saturate the sample”. Seems to be a misconception

Reply: Thanks for your advice. We have made changes to the entire sentence, which can be seen in line 12-13 of page 7. “activated to remove surface impurities under Ar for 1 h at 200 °C,”

25. Please, describe the gasification chamber used to feed the toluene

Reply: Thanks for your advice. We have added the corresponding description in line 11-12 of page 7. “the catalyst was squeezed into a thin slice in the DRIFT cell and tested in situ using absorbance”

26. How did the authors ensure that there was no channeling in the catalytic bed? What is the height of the 100 mg catalytic bed in a 2 cm diameter reactor?

Reply: Thanks for your advice. According to the questions you raised, We have added the corresponding description in line 19-20 of page 7. “100 mg of catalyst was packed into a 10 cm long quartz tube with an inner diameter of 2 cm,” and in line 1 of page 8. “(Gas flow was controlled by a gas flowmeter)”

27. Please, declare the details of the GC detectors and columns.

Reply: Thanks for your advice. We have added the corresponding description in line 3-6 of page 8. “(Detection conditions were as follows: high purity N₂ was used as carrier gas, flame ion detector (FID) was used, column temperature was 150 ℃, gasification chamber temperature was 250 ℃, detector temperature was 250 ℃, the chromatographic column was capillary column)”

28. Please, declare the units of CO₂concentration

Reply: Thanks for your advice. We have added the corresponding description in line 12 of page 8. “(ppm)”

29. The comparison of peak intensity should be done using normalized intensities. Is not really obvious that the peak intensity of Co₃O₄is larger in samples prepared at higher temperature. The particle size of metal oxide cluster could be estimated from the XRD patterns as well.

Reply: Thanks for your advice. According to the questions you raised, We have added the corresponding description in line 8-15 of page 9. “From Fig. 1, it was obvious that the peak intensity and width of Co₃O₄ are enhanced and narrowed with the increased of air calcination temperature, indicating that with the increased of calcination temperature, O₂ oxidized the metal clusters in the Co-BTC skeleton more fully, resulting in better crystallization of Co₃O₄ and the formation of metal oxide clusters with larger particle size. By comparing the diffraction peaks of the metal oxides formed by different calcination atmosphere, it can be found that the oxides calcined with N₂ had lower peak intensities and larger peak widths,”

30. The affirmation made in lines 168-169 about the influence of calcination conditions on pore structure should be discussed and supported by the results.

Reply: Thanks for your advice. We have added the corresponding description in line 12-16 of page 10. “It could be found that the oxides calcined under N₂ conditions had a larger specific surface area, where Co₃O₄-N₂ had the largest specific surface area (161.4 mg/m²), which was attributed to the carbonization of the organic ligands in Co-BTC under N₂ condition, so that the material could maintain the original skeletal structure.”

31. The plots of pore size distribution should be included in the Supplementary Information together with the isotherms

Reply: Thanks for your advice. We have added a plot of the particle size distribution of the catalysts to the Supplementary data, which can be viewed in Fig. S2 (c).

32. Line 179-180. Which result support the affirmation “the pore surface was found to gradually became smooth and also showed signs of expansion”? What is the “pore surface”?

Reply: Thanks for your advice. According to the questions you raised, We have added the corresponding description in line 4 of page 11. “By observing Fig. 2(a), (b), and (c)” and in line 10-11 of page 11. “which was consistent with XRD and BET characterization results.”, and changed “pore surface” to “surface of the grains” in lines 7, Page 11.

33. Table 2 and 3. The Co³⁺/Co²⁺and O_ads/O_lattratios should not have units

Reply: Thanks for your advice. According to the questions you raised, We have made changes to the contents of Table 2 and 3

34. Why the surface composition of the samples Co₃O₄-550 and Co₃O₄-450 is not in the results?

Reply: Thanks for your advice. Since the content of Co³⁺ and O_ads in Co₃O₄-550 and Co₃O₄-450 was not dominant, it is not discussed in detail, but the corresponding description is also given line 12 of page 13 (Co₃O₄-N₂ < Co₃O₄-550 < Co₃O₄-450 < Co₃O₄-350 ≈ Co₃O₄-N₂-O₂) and lines 8-10 of page 13 (“It could be seen that the ratio of O_ads/O_latt decreased as the calcination temperature of the material increased, indicating that high temperature calcination was not conducive to the accumulation of adsorbed oxygen in the material”) of the paper

35. Line 241. Which XRD results are consistent with the particle size increasing

Reply: Thanks for your advice. According to the questions you raised, We have added the corresponding description in lines 3-10 of page 15. “From Fig. 4 (a) it could be seen that the reduction peak of the catalyst gradually moved towards the high temperature region as the calcination temperature increased, indicating that the reduction effect of the material deteriorated under high temperature calcination conditions, which may be related to the particle size of the catalyst, and the XRD results indicated that the particle size of the catalyst becomes larger as the calcination temperature increased. It has been reported in the literature that the reduction peak tended to moved towards the high temperature region as the particle size increased”

36. The comparison made in Figure 5c and 5d must be preform using reaction rate as the parameter to stablish the efficiency of each catalyst. Is a mistake to use the conversion to compare the material

Reply: Thanks for your advice. According to the questions you raised, We have deleted relevant descriptions in Figure 5c and Figure 5d

37. Line 261-262. To support the affirmation “the internal grains of the catalyst tended to agglomerated” the authors should include the particle size of clusters in the catalysts.

Reply: Thanks for your advice. According to the questions you raised, We changed the corresponding description in line 13 of page 16. “the particle size of catalyst gradually increases”.

38. Line 270. The comparison among the catalysts could be done in a plot of the T_50%and/or T_90%to make it easy visually

Reply: Thanks for your advice. According to the questions you raised, We have modified Figure 5a and 5b. The corresponding temperatures of catalysts T₁₀, T₅₀ and T₉₀ were listed in Table S1.

39. Line 310. Please, write “stream” instead “steam”.

Reply: Thanks for your advice. We’ve changed “steam” to “stream” in Lines 1, Page 20.

40. The apparent activation energies could be easily calculated from the data. Its values should be included in the discussion.

Reply: Thanks for your advice. We have added Figure 5c (Arrhenius plots ) on page 19, and added the corresponding description in line 13-19 of page 17. “The apparent activation energy (E_a) of the catalysts were calculated by applying the linear fitting of Arrhenius plots at a conversion less than 20% (show in Fig. 5 (c)). The calculated results are listed in Table S1 in the following order: Co₃O₄-N₂-O₂ (39.65 kJ·mol−1) ＜ Co₃O₄-350 (42.06 kJ·mol−1) ＜ Co₃O₄-450 (47.84 kJ·mol−1) ＜ Co₃O₄-N₂  (67.27 kJ·mol−1) ＜ Co₃O₄-550  (70.43 kJ·mol−1). The lowest E_a corresponds to the easiest possibility of toluene decomposition. The result is consistent with that of toluene conversion.”

41. Why Co₃O₄-P was not included in any previous characterization? At least in XRD to confirm the Co phases

Reply: Thanks for your advice. The main purpose of this paper is not to analyze the structure of Co₃O₄-P, but to compare Co₃O₄-P with Co₃O₄-N₂-O₂ to highlight the advantages of catalytic activity and mineralization rate of Co₃O₄-N₂-O₂. So there was no characterization test for Co₃O₄-P.

42. There are no results to confirm that the mechanism fit to a L-H model. To do that, the authors must fit the experimental data to all the three models mentioned and show, mathematically, the better fit to L-H model.

Reply: Thanks for your advice. The mechanism of toluene degradation by Co₃O₄-N₂-O₂ was verified by testing the In situ DRIFTS spectra of Co₃O₄-N₂-O₂ under different atmospheric conditions (Fig .6 (a) and (b)), the same method was also documented in the literature [1]. The corresponding explanation could be found on page 20 and 21 of the article.

[1] Lei J, Wang S, Li J, Xu Y, Li S: Different effect of Y (Y = Cu, Mn, Fe, Ni) doping on Co₃O₄ derived from Co-MOF for toluene catalytic destruction. Chemical Engineering Science 2022, 251:117436.

43. How the authors are sure that “benzaldehyde, benzoic acid, bicarbonate and carbonate species were intermediate products in the degradation process of toluene” and not just spectators in the catalyst surface?

Reply: Thanks for your advice. During In situ DRIFTS testing, we activate the catalyst to remove surface impurities under Ar for 1 h at 200 °C (in lines 12-13, Page 7.). Therefore, no characteristic peak corresponding to benzaldehyde, benzoic acid, bicarbonate and carbonate species were found in Fig. 6 (a), and Fig. 6 (b) at 30 °C. Observation of Fig. 6 (b) showed that benzaldehyde, benzoic acid, bicarbonate and carbonate species was produced during the subsequent reaction.

44. Figure 7. The steps shown lack experimental support and the methodology used to show them is not adequate. As a suggestion to the authors I leave this reference.

Reply: Thanks for your advice. We have changed Fig .7, and the corresponding description has been changed in lines 8-16, Page 21. “ From the above analysis it was clear that the catalytic oxidation effect of the lattice oxygen of the catalyst on toluene adsorbed on the catalyst surface was not significant. The mechanism of toluene degradation by Co₃O₄-N₂-O₂ catalyst was in accordance with the L-H model and the main processes were shown in Fig. 7. The toluene molecules were first adsorbed on the catalyst surface and then gradually degraded by the interaction with the adsorbed oxygen. With the increase of temperature, toluene molecules were gradually oxidized into benzaldehyde, benzoic acid, bicarbonate and carbonate species and finally decomposed into small molecules of CO₂ and H₂O.”.

Author Response File: Author Response.pdf

Reviewer 2 Report

In the present manuscript Co3O4 NP were synthesized derived from Co-BTC. The effect of calcination temperature and atmosphere on the physico-chemical properties of the final material was studied. The authors found that the material prepared under N2 and O2 atmosphere showed a higher metal dispersion and highest catalytic activity in the oxidation of toluene. The reaction mechanism was followed by DRIFT. According to the authors three steps are involved in the reaction path: the adsorption of oxygen molecules onto oxygen vacancies on the catalyst surface forming activated oxygen species, adsorption of toluene molecules onto Co3+ active sites and the reaction between adsorbed toluene molecules and reactive oxygen species. But, no experimental evidence of the formation of active oxygen species, nor adsorption of toluene on Co3+ sites are given. This need to be revised carefully by the authors including experimental evidences of the reaction mechanism. On the other hand, IR data need to be revised. For example, adsorbed benzaldehyde and benzoic acid present an IR band of  C=O, which are missed in the IR spectra.  The authors should analyse reference spectra in order to assign IR bands.

On the other hand, additional information need to be carefully analysed. For instance

Determination of the Co3O4 particle size from the XRD pattern need to be included and discussed.

XPS spectra of Figure 3 at the Co2p3/2 core line of the Co3O4-N2 sample need to be revised. The separation between satellite peak marked in blue and the main peak is not consistent with that of the other samples.

More details in the experimental section regarding XPS analysis need to be included, for instance charge correction, pass energy, X-Ray power, etc.

The decrease in the conversion at increasing reactant concentration has nothing to do with a detrimental reactant concentration. The authors should remove that comment (line 280-282).  They should calculate the number of molecules converted by mass of catalysts.

Some comments  are difficult to understand , for example line 293-295: “Where the Co3O4-N2-O2 nanocatalyst had toluene  degradation efficiencies of 234°C and 244°C for T50 and T90, respectively, which were in general agreement with the toluene mineralization efficiencies of 236°C and 245°C corresponding to T50 and T90.”; line 247-248 “ the reduction peak of Co3O4-N2 was very weak, which was a good indication that the metal Co in Co-BTC calcined under N2 was an incomplete oxidation process and thus lacked sufficient Co3+ active sites”. Please reformulate those comments.

In general, the manuscript need to be revised carefully. Also the authors need to assess what is the impact of their work respect to other studies in the literature that makes the work relevant for publication. At the present I cannot find any new insight respect to the state of the art.

Author Response

Reviewer #2: In the present manuscript Co₃O₄ NP were synthesized derived from Co-BTC. The effect of calcination temperature and atmosphere on the physico-chemical properties of the final material was studied. The authors found that the material prepared under N₂ and O₂ atmosphere showed a higher metal dispersion and highest catalytic activity in the oxidation of toluene. The reaction mechanism was followed by DRIFT. According to the authors three steps are involved in the reaction path: the adsorption of oxygen molecules onto oxygen vacancies on the catalyst surface forming activated oxygen species, adsorption of toluene molecules onto Co³⁺ active sites and the reaction between adsorbed toluene molecules and reactive oxygen species. But, no experimental evidence of the formation of active oxygen species, nor adsorption of toluene on Co³⁺ sites are given. This need to be revised carefully by the authors including experimental evidences of the reaction mechanism. On the other hand, IR data need to be revised. For example, adsorbed benzaldehyde and benzoic acid present an IR band of  C=O, which are missed in the IR spectra.  The authors should analyse reference spectra in order to assign IR bands.

Reply: Thanks for your advice. We have changed Fig .7, and the corresponding description has been changed in lines 8-16, Page 21. “ From the above analysis it was clear that the catalytic oxidation effect of the lattice oxygen of the catalyst on toluene adsorbed on the catalyst surface was not significant. The mechanism of toluene degradation by Co₃O₄-N₂-O₂ catalyst was in accordance with the L-H model and the main processes were shown in Fig. 7. The toluene molecules were first adsorbed on the catalyst surface and then gradually degraded by the interaction with the adsorbed oxygen. With the increase of temperature, toluene molecules were gradually oxidized into benzaldehyde, benzoic acid, bicarbonate and carbonate species and finally decomposed into small molecules of CO₂ and H₂O.”.

Benzaldehyde and benzoic acid In situ DRIFTS spectra was identified according to the description in reference [2], which was described as follows: “Above 200 °C, the obviously new bands at 2840 and 2721 cm⁻¹are possibly attributed to adsorbed benzaldehyde, although the band at around 1710 cm⁻¹ is missed or inconspicuous. The bands at 1563, 1547 and 1391 cm⁻¹ are characteristic of typical carboxylate group, indicating the formation of benzoate species that have been identified as key intermediates in toluene oxidation.”

[2] Chen X, Chen X, Yu E, Cai S, Jia H, Chen J, Liang P: In situ pyrolysis of Ce-MOF to prepare CeO₂ catalyst with obviously improved catalytic performance for toluene combustion. Chemical Engineering Journal 2018, 344:469-479.

2. Determination of the Co₃O₄ particle size from the XRD pattern need to be included and discussed.

Reply: Thanks for your advice. According to the questions you raised, we have added the corresponding description in lines 8-15 of page 9. “From Fig. 1, it was obvious that the peak intensity and width of Co₃O₄ are enhanced and narrowed with the increased of air calcination temperature, indicating that with the increased of calcination temperature, O₂ oxidized the metal clusters in the Co-BTC skeleton more fully, resulting in better crystallization of Co₃O₄ and the formation of metal oxide clusters with larger particle size. By comparing the diffraction peaks of the metal oxides formed by different calcination atmosphere, it can be found that the oxides calcined with N₂ had lower peak intensities and larger peak widths,”

3. XPS spectra of Figure 3 at the Co 2p3/2 core line of the Co₃O₄-N₂ sample need to be revised. The separation between satellite peak marked in blue and the main peak is not consistent with that of the other samples.

Reply: Thanks for your advice. In the fitting process of XPS, we did find that Co₃O₄-N₂ catalyst would have a certain degree of deviation compared with other catalysts. It can be seen from Fig .1 that Co in Co₃O₄-N₂ exists in the form of CoO, while other catalysts exist in the form of Co₃O₄. It may be that the difference of Co morphology of catalyst leads to the difference of binding energy of XPS.

4. More details in the experimental section regarding XPS analysis need to be included, for instance charge correction, pass energy, X-Ray power, etc

Reply: Thanks for your advice. We have added the corresponding description in lines 1-4 of page 7. “X-ray photoelectron spectroscopy (XPS) analysis was performed on an Thermo Fisher Scientific spectrophotometer with Al Kα(1486.6 eV) as an X-ray excitation source. The survey and narrow scans were done at pass energies of 100 eV and 30 eV, respectively. The C1 s was taken as reference and assigned binding energy of 284.8 eV.”

5. The decrease in the conversion at increasing reactant concentration has nothing to do with a detrimental reactant concentration. The authors should remove that comment (line 280-282).  They should calculate the number of molecules converted by mass of catalysts.

Reply: Thanks for your advice. We have removed lines 280-282.

6. Some comments  are difficult to understand , for example line 293-295: “Where the Co₃O₄-N₂-O₂ nanocatalyst had toluene  degradation efficiencies of 234°C and 244°C for T₅₀ and T₉₀, respectively, which were in general agreement with the toluene mineralization efficiencies of 236°C and 245°C corresponding to T₅₀ and T_90.”; line 247-248 “ the reduction peak of Co₃O₄-N₂ was very weak, which was a good indication that the metal Co in Co-BTC calcined under N₂ was an incomplete oxidation process and thus lacked sufficient Co³⁺ active sites”. Please reformulate those comments

Reply: Thanks for your advice. We’ve changed “Where the Co₃O₄-N₂-O₂ nanocatalyst had toluene  degradation efficiencies of 234°C and 244°C for T₅₀ and T₉₀, respectively, which were in general agreement with the toluene mineralization efficiencies of 236°C and 245°C corresponding to T₅₀ and T₉₀” to “The toluene degradation efficiencies of Co₃O₄-N₂-O₂ catalysts at 50% and 90% were 234 ℃ and 244 ℃, respectively, which were basically consistent with the temperatures corresponding to toluene mineralization efficiencies at 50% and 90% (236 ℃ and 245 ℃).” in Lines 4-7, Page 18.

We’ve changed “the reduction peak of Co₃O₄-N₂-O₂ was very weak, which was a good indication that the metal Co in Co-BTC calcined under N₂ was an incomplete oxidation process and thus lacked sufficient Co³⁺ active sites” to “Compared with Co₃O₄-N₂-O₂, the reduction peak of Co₃O₄-N₂ was very weak, because the Co in Co₃O₄-N₂ catalyst existed mainly in the form of CoO and thus lacked sufficient Co³⁺ active sites.” in Lines 15-18, Page 15.

7. In general, the manuscript need to be revised carefully. Also the authors need to assess what is the impact of their work respect to other studies in the literature that makes the work relevant for publication. At the present I cannot find any new insight respect to the state of the art.

Reply: Thanks for your advice. We have added the corresponding description in line 17-21 of page 4. “However, most of the current studies were only limited to investigating the effect of calcination temperature on the catalysts, or the effect of calcination atmosphere on the catalysts, and the synergistic influence of calcination temperature and atmosphere on physical and chemical properties of materials has not been comprehensively analyzed.” and in line 1-6 of page 5. “To explore the influence mechanism of the Co₃O₄ catalysts formed by calcination temperature and calcination atmosphere on the degradation activity of toluene. in order to find the ideal synthetic route for the synthesis of Co₃O₄ materials using Co-BTC as the precursor, and to provide a reference for the synthesis process of metal oxides derived from MOFs.”

Author Response File: Author Response.pdf

Reviewer 3 Report

In the manuscript, Z. You et al. report on preparation of Co-MOF under different calcination methods and studied their activity for toluene oxidation. The work is good, manuscript is well prepared except a few grammatical mistakes and materials characterization seems to be well done. However, some changes/upgrades must be addressed prior to publication:

1.      Title should be revised, it is not clear.

2.      There are some grammar, typo, and style mistakes.

3.      How did the authors optimize the oxidation reaction condition and ratio of the reagent?

4.      More recent references should be added and discussed.

Author Response

Reviewer #3: In the manuscript, Z. You et al. report on preparation of Co-MOF under different calcination methods and studied their activity for toluene oxidation. The work is good, manuscript is well prepared except a few grammatical mistakes and materials characterization seems to be well done. However, some changes/upgrades must be addressed prior to publication:

1. Title should be revised, it is not clear.

Reply: Thanks for your advice. We have revised the title of the paper, the revised title in Lines 1-2, Page 1, “Effect of calcination temperature and calcination atmosphere on the performance of Co₃O₄ catalysts for the catalytic oxidation of toluene”.

2. There are some grammar, typo, and style mistakes.

Reply: Thanks for your advice. We have changed the corresponding problems

3. How did the authors optimize the oxidation reaction condition and ratio of the reagent?

Reply: Thanks for your advice. The oxidation reaction condition and ratio of the reagent were determined by continuous experiments in the early stage of the experiment.

4.  More recent references should be added and discussed.

Reply: Thanks for your advice. We add three recent references to the revised manuscript, such as [13], [14] and [38].

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments to the revised version of the article: “Effect of calcination temperature and calcination atmosphere on the performance of Co₃O₄ catalysts for the catalytic oxidation of toluene”.

The authors include the isotherms and particle size distribution (PZD) in the SI. Also included the methods used to estimate the specific surface area and the PZD. However, the info about which device was used to perform the test is missing, as well as the technique (adsorption-desorption of N₂ maybe).

The way to supply the toluene still not included in the experimental section. Since the mix of a liquid in a gas stream is one of the most challenging issues in experimental heterogeneous catalysis, this info is mandatory to ensure the reproducibility of the results. If the reproducibility is not possible, the publication of the manuscript is not possible.

Another problem with the reproducibility is the ratio catalyst bed height:reactor diameter. The authors reply to the point 26 in the previous comments does not show that the authors understand the problem, which leads to the conclusion that the configuration of the reactor-catalyst system does not meet the requirements for meaningful results that exclude transport problems such as channeling.

The reply to point 29 in the previous comments does not address the above-mentioned problem. The comparison of non-normalized patterns does not make sense. On the other hand, the mention of narrower peaks must be supported by FWMH values.

As for the analysis of the FTIR results, the fact that the peaks related to benzaldehyde, benzoic acid, bicarbonate and carbonate species are not present under Ar flow only shows that these compounds are produced under reaction conditions; it says nothing about what role they play in the reaction itself. If the authors want to show that their results are explained by an L-H mechanism, they must at least establish the elementary steps that this mechanism follows (Figure 7 is just a reaction pathway). If they want to demonstrate that it is indeed an L-H mechanism, they must propose an L-H model, starting by showing which of the surface intermediates is dominant, and fit their data to this model.

For above reasons, my suggestion is to reject the manuscript.

Author Response

Dear editor and revewers:

Thank you very much for giving us the opportunity to revise our manuscript. We appreciate the reviewers for your constructive comments and suggestions on our article entitled " Catalytic oxidation of Co₃O₄ nanocatalysts derived from Co-MOF for Toluene：effect of calcination temperature and calcination methods" (processes-2274190).

We have studied the comments carefully and made corrections that we hope meet with approval. The main corrections have been marked in red in the revised manuscript. Now, we will reply the questions point by point.

 

Reviewers' comments:

 

Reviewer #1:

1. The authors include the isotherms and particle size distribution (PZD) in the SI. Also included the methods used to estimate the specific surface area and the PZD. However, the info about which device was used to perform the test is missing, as well as the technique (adsorption-desorption of N₂maybe)

Reply: Thanks for your advice. We have added the corresponding description in line 13-15 of page 6. “The catalysts’ specific surface area, pore volume and pore size distribution were obtained by Micromeritics ASAP 2460 Automated Gas Sorption analyzer.”.

2. The way to supply the toluene still not included in the experimental section. Since the mix of a liquid in a gas stream is one of the most challenging issues in experimental heterogeneous catalysis, this info is mandatory to ensure the reproducibility of the results. If the reproducibility is not possible, the publication of the manuscript is not possible

Reply: Thanks for your advice. We have added the corresponding description in line 19-20 of page 7. “Toluene was injected from single-channel syringe pump (SPLab01) into the gasification chamber”. The single channel syringe pump is shown in Fig. 1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 1. single-channel syringe pump (SPLab01)

3. Another problem with the reproducibility is the ratio catalyst bed height:reactor diameter. The authors reply to the point 26 in the previous comments does not show that the authors understand the problem, which leads to the conclusion that the configuration of the reactor-catalyst system does not meet the requirements for meaningful results that exclude transport problems such as channeling.

Reply: Thanks for your advice. The height of our catalytic bed was approximately 1 mm. With regard to repeatability, the catalyst was tested five times using the same apparatus and method, and the results are shown in Fig. 2. It was found that 3 of the 5 test results were completely consistent, and that the results of all 5 tests would tend to be consistent as the temperature increased.

 

 

 

 

 

Fig. 2. 5 times repeatability test for catalyst

4. The reply to point 29 in the previous comments does not address the above-mentioned problem. The comparison of non-normalized patterns does not make sense. On the other hand, the mention of narrower peaks must be supported by FWMH values.

Reply: Thanks for your advice. Peak height and FWHM data for XRD diffraction peaks have been supplemented in the supplementary data (Table S1 and Table S2).

5. As for the analysis of the FTIR results, the fact that the peaks related to benzaldehyde, benzoic acid, bicarbonate and carbonate species are not present under Ar flow only shows that these compounds are produced under reaction conditions; it says nothing about what role they play in the reaction itself. If the authors want to show that their results are explained by an L-H mechanism, they must at least establish the elementary steps that this mechanism follows (Figure 7 is just a reaction pathway). If they want to demonstrate that it is indeed an L-H mechanism, they must propose an L-H model, starting by showing which of the surface intermediates is dominant, and fit their data to this model.

Reply: Thanks for your advice. The description of the L-H mechanism in the paper has been removed. The corresponding description has been changed in lines 9-16, Page 20. “From the above analysis, it was clear that the toluene molecules in the air were adsorbed on the surface of the catalyst, and catalytic oxidation effect of the lattice oxygen of the catalyst on toluene adsorbed on the catalyst surface was not significant. The degradation process of toluene molecules were shown in Fig. 7. The toluene molecules were first adsorbed on the catalyst surface and then gradually degraded by the interaction with the adsorbed oxygen. With the increase of temperature, toluene molecules were gradually oxidized into benzaldehyde, benzoic acid, bicarbonate and carbonate species and finally decomposed into small molecules of CO₂ and H₂O”.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manucript is more consistent now and suitable for publication. However the authros should determnine the particle size from XRD using the scherrer equation. It can be added for instance in table 1. It is better to have values instead of a qualitative description of small-big particle sizes. 

Author Response

Response to Reviewers

Dear editor and revewers:

Thank you very much for giving us the opportunity to revise our manuscript. We appreciate the reviewers for your constructive comments and suggestions on our article entitled " Catalytic oxidation of Co₃O₄ nanocatalysts derived from Co-MOF for Toluene：effect of calcination temperature and calcination methods" (processes-2274190).

We have studied the comments carefully and made corrections that we hope meet with approval. The main corrections have been marked in red in the revised manuscript. Now, we will reply the questions point by point.

Reviewer #2:

1. The manucript is more consistent now and suitable for publication. However the authros should determnine the particle size from XRD using the scherrer equation. It can be added for instance in table 1. It is better to have values instead of a qualitative description of small-big particle sizes. 

Reply: Thanks for your advice. Peak height and FWHM data for XRD diffraction peaks have been supplemented in the supplementary data (Table S1 and Table S2).

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

The effort made by the authors is admirable and the correction of the points made has been done in a satisfactory manner. However, the fact that the catalytic bed is 1 mm wide invalidates the result of the experiments. The reproducibility of the tests does not mean that they meet the required standards with respect to mass and heat transfer and catalyst bed geometry. For this reason, I must suggest that this article not be published in the journal.

As a suggestion to authors for future work I invite them to check the feasibility of their experiments on this site:

https://www.eurokin.org/wp-content/uploads/webtool/EUROKIN_fixed-bed_html.htm